Mode-locked lasers are particularly useful for the generation of ultrashort optical pulses, for example pulses of widths in the picosecond and subpicosecond range, and typically of high energy. Semiconductive saturable absorbers are advantageously employed in such mode-locked lasers as Bragg-reflector mirrors.
A saturable absorber absorbs all weak incident radiation but when the intensity of the radiation is built up to a sufficiently high level, termed the saturation intensity, the saturable absorber is bleached and becomes transparent, and then incident radiation is permitted passage with relatively low attenuation. A saturable Bragg absorber is a non-linear element that acts on incident radiation as a shutter whose opacity changes as a function of the intensity of the incident radiation at a particular wavelength. When used in a laser as a Bragg-reflector mirror, the saturable Bragg absorber acts in its opaque state as a low-loss reflector of the incident stimulated emission of the laser, and so can serve as one end wall of its resonant cavity.
U.S. Pat. No. 5,627,854 that issued to applicant W.H. Knox on May 6, 1997 describes a mode-locked laser in which the mode-locking element is a saturable Bragg reflector incorporating a quantum well within the one or more low-index layers the top of a stack advantageously of at least twenty of quarter optical path wave-length layers alternately of low and high index of refraction. The resulting non-linear reflector provides an intensity-dependent response that permits it to be used for saturable absorption directly in the main oscillating cavity of the laser.
In many applications the gain medium of the laser is quite small allowing the absorber material, the quantum wells, to be few in number, and in such applications the absorber material little influences the optical design of the dielectric Bragg-reflector mirror in which the absorber material is placed. However for a high gain laser where a large modulation depth is desired for the saturable Bragg reflector, the prior art saturable Bragg reflector design generally requires expanding the number of absorber layers that need to be incorporated, often to ten or more. In this case the absorber layer section becomes a significant factor in the mirror design and tends to introduce significant optical scattering because of the dielectric discontinuities introduced by the absorber layers. In fact when many quantum wells are placed inside the dielectric quarter optical path wavelength layers, there must be increased the total number of quarter wavelength layers and this leads to larger scattering penalties and non-optimized positioning of the quantum wells within the stack.
The present invention seeks a more efficient way of introducing the quantum well absorber layers into the saturable Bragg reflectors for use in mode-locked lasers.